Wet-weather accidents
What can road authorifies do about them?
5HJi.P~I.P
INSTITUTE FOR ROAD SAF ETY RESEARCH SWOV
The Institute
The Institute for Road Safety Research SWOV was founded In 1962. Its object is, on the bas's of sc'entific research ,to supply the authonties with data for measures aiming at promoting road safety .The Information obtained from thiS scientific research IS disseminated by SWOV ,either as individual publications ,o'ras articles in periodicals or Via other
communication media.
SWOV's Council consls:t; of representatives of various Ministries, of industry and of leading social institutions.
The Bureau is managed by E.Asmussen, SWOV's Director .Its departments include a.o . Research Co ordination, Research Services .Pre 'Crash Research ,Crash and Post crash Research, Methods and Techniques, and Information.
lhe booklet Wet-weather accidents' has been comp led by SWOV's Information Department (M.P.M. Mathljssen and R.Maas) .
lhe bas,c mate I'BI was p IOvlded by D Stoelhors t(Provlncial Publ'c Works Department of Noord-Brabant) and SWOV workers J Doornekamp, L H.M Schldsser and A G Welleman. Photographs by:ANP foto (cover, p 8) . This brochure can be obtained free of charge from Institute for Road Safety Research SWOV, Information Depart -ment, P.O.Box 71, 2270 AB Voorburg , The Netherlands.
Foreword
The hstltute for Road Safety Research SWOV is conducting large sca
e
research intro traffic hal2a rds in the prov nce 0 f Noord Brabant br that prov nces
CounCl land the Minister ofTransport and Waterways,
Besides the spec'flc problems in Noord -Brabant ,research IS also being carried out I'nto more general problems, Use is being made of available knowledge obtained
i"l
earher research by SWOV , from literature studies or from practical experience, One such problem 'I; the occurrence of accidents on wet road surfaces ,An Interim report on th'l; has already been made .Although the report is focused on the sltuat bn as regards Noo rtJ-Brabant provincia\lroads ,much of the Information IS a so useful for road authonh:ls in other provinces, Since the situation in Noord Brabant is unlikely to differ much from that in the other p !tlvlnCes of The Netherlands, This IS Ind't:ated by the number of wet surface accidents as compared with the btal number of accidents ,ThiS ratio IS about the same In Noord Brabant as 'h the country as a whole,These considerations have Induced SWOV to pUblISh this booklet ,special y intended for road authonties ' They are the very ones who can make an effec -tive contnbutlon towards controlling wet weather accidents, lhe booklet, With the list of recommended literature
at the end IS Intended to assist them I'n this ,Moreover ,SWOV at all times we'~ comes questions or comments from readers,
To keep the booklet readable, extensive sCientific arguments supporting the contents have been omitted, Anyone Interested In these can find them in a series of three reports:Wet weather acc'tlents I, Illand Ill, SWOV R-79 27, R -79 28 and R-79-29 ,( Only In Dutch, with Enghsh summary)
E Asmussen
Dl'recto r hstitute for Road Safe
\t
Research SWOVAccidents
on wet
road
surfaces:
extent
and causes
The likelihood of being involved In an acc 'dent is over twice as great on a wet road surface as on a dry one. The In -creased risk is the greatest for private car occupants; on a wet road they are nea fy three times as likely to have an accident as on a dry one .Moreover, this road tJse!rcategory has the most casualties on wet road surfaces in
Deaths Pedestrians 1974/1976 Total 1,207 On wet roads 333 Risk-increasing factor 2.2 Injured Pedestrians 1974/1976 Total 19,336 On wet roads 4,743 Risk increasing factor 18
absolute terms :948 car deaths out of a total of 2004 wet weather accident
deaths in the years 197411976.
Calculat'b nof the risk increasing factor was based on roads being wet 150
6
ofthe time.
The higher risk on wet road sur bces IS partly explaihed by vehicles SkiDding. An extensive study of this subject has
Cyclists Moped Car
riDerS occupants
1,424 1,102 3,013
329 261 948
1.7 18 2.6
-Cyclists Moped Car
riders occupants
321497 61 ,i09 62,940
7562 14808 21,465
11 18 2.9
been made by the SWOV Working group on 'lYres, Road Surfaces and Skidding AccIDents' .
When there IS a film of water on the road, contact between tyre and surface is impeded and there IS less friction. This makes 'tl more difficult for a drlve!r to keep his vehl'ce under control by steering and braking and the accident
Others Total 553 7,299 133 2,004 1 8 2.1 Others Total 12,005 188,487 2,629 51207 1.6 2.1
risk will Increase. Further analyses have shown that the type of road, traffiC volume and the proportion of goods vehicles in the overall traffic flow also play a part in wet-weather accidents. But friction has the greatest influence of all.
All factors governing friCtion between tyre and wet-road surface will influence the oCcurrence of accidents. These factors are: road-surface quality, vehiCle speed, type of tyre, depth of tread and puddle forming due to un-evenness, changes in banking, Inclines
or
poor water removal to the verge .The first of these factors -road surface quality -relates to micro and macro road surface roughness .Micro -roughness can be defined as un -evenness of 0.1 to 0 5 mm, and macro -roughness from 1 to 20 mm·The thicker the water film on the road becomes, the less friCtion there IS between tyre and surface. Even with 2 to 3 mm water on the road the available friction is drastically reduced, espe -clally at high vehicle speeds and slight macro Toughness of the surfa Q:l . The other factors then hardly play any role.
It IS therefore of the greate~· Impor -tance to prevent such films being formed.
If there IS only a thin film of wal\3r on the road, the friction at low speeds is dete ttnli~d mainly by the m ~ro
-roughness of the surface .As speeds Increase the friction will decrease, but less so the better the micro Toughness is .On road surfaces with inferlo'rmacro -roughness, especially the influence of the depth of the tyre tread is an important factor. Water in the contact area between tyre and road surface 15 then largely expelled via the tread. Hence, the friction may still be great enough .SWOV measurements have shown that about one and a half per cent of private cars in The Netherlands have one or more tyres with a tread less than 1 mm thick (the legal norm) .About five per cent have one or more tyres With a tread less than
2
mm 'Once enough friction IS available, this is nog guarantee that it is fully utilised Various additional measures are POSSI'-ble to do thiS, They may be focused on people (by influenCing behaviou rby slgnalhng ,speed limits and driver training) or on vehiCles (by means of effective braking systems With antl' -locking deVices) .
Skidding is one of the major causes of acclClents on wet road surfaiQ:lS .
The road auth
o
riti
es
' tas;
ks
The previous section has indicated that road-surface quality is of the greatest importance in controlling skidding accidents. It is the road authority that can directly influence.the principal factors governing friction between tyre and road surface. Before the road authority takes action to reduce wet-weather accidents it will have to weigh the cost and benefits against the cost and benefits of other road-safety measures. In order to employ available resources as effectively as possible, its preference w~1 be for measures giving the best cost/benefit ratio. As all the information really needed for such decisions is not actually available, the policy wiU have to be based on limited data. The first question that has to be answered is whether wet-weather accidents are to form an aspect of policy or not. It is important to know the maximum effect of measures aimed at reducing accidents on wet road sur-faces. Bas.k;ally, ~ I'S possible to reduce the accident hazard on wet roads to the level of that on dry ones ,so that the total number of accidents can be reduced at most by 12 to 15%.lf the road authority de'cides that wet weather accidents should form an aspect of its road -safety policy, it will have to set priorities. It can then:
- Establish norms for a number of factors influencing the friction between tyre and wet road-surfaces.
- Trace locations where the number of wet-weather accidents is comparative-ly high.
Establishing norms
The road authority can establish norms for a number of factors influencing tyrel surface frt;tions. It can measure the actu aI condition of a road surface compared with the norm, and can then set priorities. The locations deviating most from the norm quality first for act bn. The various factors for which the road authority can establish no rms are: - skidding resistance;
- variations in skidding resistance by time and place;
- reduction in friction as speeds increase;
- the thickness of the water film on the road.
At present a guide value exists only for the skidding resistance level. though only for national highways. The minl' -mum level establishedforthese roads is 0.51 in measurements by th e Dutch State Road Laboratory method, In 1974, over ten per cent of na1'lonal highways were found not to meet this
minimum standard .If the same norm
had applied to the other roads I'n this country, thl'rty to forty percent of them would not have met 'It .There is unh'kely to have been any subs tan tial improve
-ment since then. There are no norms at all for the other fac tors. The Dutch State Road Labora tory, however, has carried out extensive research I·nto the in-fluence of the depth of the ~lm of water on the road. This showed th at tyrel surface friction is already very greatly reduced with a film 2 to 3 mm thl'ck, It is therefore recommended taking steps to ensure that such f .ms of water can
-not form.
n
establish ing norms, the road authori -ty will have to take various aspects of the t raff k; system into account, such as . safety, smooth f bw, comfort, cost and harm to the environment .In order to weigh all these aspects against one another, a system is being developed, known as rational road management. Rational road management is at present in the stage of bUilding a theory and initial experiments. ~ may take some time before the system is fully operational.Two approaches are possible in setting priorities by reference to norms. The first is based on the driver's pattern of expectations .If he is able to make certain braking and steering manoeuvres on a good road surface, hie will tend to think that he can do the same on other surfaces. Moreover, differences in the quality of various road surfaces are often beyond his powers of
observation. In order to meet his pattern of expectations, all road sur -faces should in theory have the same uniform characteristics. But in practice,
such an approach will be no easy matter.
But one can assume instead that the need to brake and steer is not the same for every category of road and every road section, At intersections, in bends, at other discontinuities and at locations with a poor view, mo re, and bolder manoeuvres will have to be made than on straight stretches of road where the view is unobstructed.
At locations of the hrst type, the surface properties will have to be better than elsewhere. This is to avoid big concen
-trations of wet weather accidents. With this approach, some locations would have to meet higher standards than others.
Tracing locations with
a
comparatively large number of wet weather acddentsThere may be various causes why a comparatively large number of wet
-weather accidents happen at a given location. For instance, at intersections
the available friction may be insufficient because a poor view necessitates bolder manoeuvres. Or may be local -ised puddles form on the surface, for
instance where the banking changes .In order to trace the causes . locations with
a comparat ~ely large number of acci-dents must be examined more closely. First of all. skidding resistance data will have to be collected ~ addition. visual
inspection Will be required. This ·lnspec
-tion is to establish what circumstances
At intersections, in bends and at places
where the view is obstructed good road'
surface quality is needed to prevent
accidents on wet road surfaces.
7
obstruct the view, what irregula rities there are in the course of the road, bends, exits, bus stops, absence of marking lines, signs or boards and so on . Such things can cause bolder
manoeuvres.
Visual inspection should take place during rainfall, so that it can also be ascertained whether there are any puddles on the road, With reference to this information, priorihes can be set and it can be decided per bcation or group of identica Ilocahons whether and, if so, what measures are required.
To sum up, the foregoing leads to the following recommendations for the road authority's porty:
In the short term (one or two years) it can trace wet weather black spots, The criterion could be the number of wet -weather accidents as compared with the total number of accidents. This would allow precisely those places to be dealt with where many wet weather accidents happen. As this relates to a small part of the roads system, the cost will be comparatively low.
In the medium-term (three to five years) it can examine whether the information gained in the short 1erm approa'Ch lends itself for drawing generalconclu' sions, By reference to this it can trace locations where greater concentrations of accidents are likely in the future.
Furthermore, it should chart road sections with the skidding resistance qualifications 'dangerous', 'very slip'
pery' and 'slippery' It can at the same time examine the resistance of ad' jOlnmg road sections and locations where skl'dding res,istance greatly de -creases in successive years. Paralle I to tl1l's it can chart locations w~h thick films ofwaterdurng rainfal l( 2t03mm). With the aid of all this information it can draw up a list of prIOrities.
In the long term (five to ten years) the road authority can derive the benefits of rational road management." weighing objective criteria of safety, comfort, smooth traffic fbw and environment, norms are established for a number of factors influencing tyre/surface friction If the actual situation is compared with the norms, it can be exam hed which locations qualify for action first
What action ca n the
road authority take?
The road authority can take steps to prevent accidents or reduce their seven'ty, Measures reducing their seven'ty are not spedfl'cally effective while the road surface is wet Guiderail structures, impact attenuators, yielding &ghting oolumns and so onfunction with accidents on dry surfaces as well. The greatest effect can thus be expected from accident'preventl'on measures Poor visibility through failing and splashing water, glare from reflected sunshine or headlamps, poore rvisibility of road markl'ngs and a reduction in frl'chonal forces between tyre and road surface are factors that may contribute to wet weather accidents Both during and after ral'nfall, these factors are Influenced adversely by the thickness of the film of water on the road surface. The road autho rity is responsible for designing, paving and maint aining a road so that puddle -forming is avoided as far as possible, Where thl's is un -avoidable, measures must be taken to keep any adverse effects to a minimumRoad design
The road designer's responsibility com . prises road geometry (Ira cl< , longItudi
-na and cross se ctions), and also provIsion of information to road users, In order to avoid wet weather a C'Cidents
as ful
y
as possIble, the road de ~gnerwd have to wat Ch a number of POInts. Geometry
The road deslgne rmust aim at not causing road users to make sudden steering movements or re·clJce theIr speeds suddenly. Places where road users often have to apply theIr brakes are: IntersectIons, eXIt lanes, places where there are regularly queues .It should be avoided sitIng such locatIons in bends or on downward inclines ,be -cause braking at such points reqUIres greater fnCtlonal forces, whereas those avaIlable are in fact sIJghte'r.Contlnuous water films on the surface must be prevented especIally at place swhere road users often have to brake or stee r.
ThIS demands a good combinatIon of longitudinal and transverse gradIents, fo rinstance where the bankIng changes .It must not be possIble for the WInd b bUIld up water at su Ch places No rmust water removal be Impeded by the thermoplastIc markIngs.
P IOvlclln g,n formatIon
Poorer v Slblhty and glare often form impedIments to road users driving on wet roads .In these cIrcumstances good
hformation on the desIgn of the road IS ve fy Important, especIally at dlscontl· -nUlt,es !bends, ungraded I·rterse Qlons ,
and so on).
InformatIon provided by clear boards, signs and markIngs enables the user to predIct the sItuatIon ahead .Thl SWIll often enable sudden brakIng or steenng to be avoIded.
•
The presence of water on the road not
only reduces available friction, but splashIng water may also greatly redu ~ vlslblitty.
Road pavement
Given the road geometry (track, longl'· tudinal and cross sections) a number 0 f areas requiring the attentl-on of the pavement designer can be I'ndicated, A good choice of materials will ensure that no water films occur, 0 rthat the
t
thickness remains tmitedh
additi on, the pavement surface can be prov'lded with such micro and macro· roughness that the adverse effects of any water film the re may sUI be on tyrel surface friction are kept to a minimum,Porous asphalt;
c
concreteThe best solution of all IS to aVOId any water film whatever, however thick ,ThiS IS possible if the water falling on the pavement is disposed of straight through the surface and stored in the upper pavement layer, This requires a top layer with a very high proport'on 0 f cavities, If the water is to be removed through the layer to the verge the cavi· ties must have open access to one another,
There is a bituminous paving mate rl'al that meets these requirements. It is usually known as porous aspha Ite concrete.
In The Netherlands this materal has been tested mainly at the Inst gna lI'on of the Road Construction Research
Centre SCW and the State Road Laboratory. Their conclusion was that porous asphaltic concrete is a material that can be wl-dely used to control water nUisance. Thl's relates not only to the forces that can be transfer red bet wee n
Thick water films may be formed
I
h
rut s and greatly '-mpede contact be weEn tyre and lOad surface.tyre and road surface. But in partIcular the huge reduction in nuisance due to splash and spray and the relat ively good reflective properties in rainy weather should be mentIoned.
As against these favourable aspects, however, there are several drawbacks.
For instance, the cavitIes gradually get clogged with dust, sand, dirt and oil residues This Jedu res draInage proper
-tl8S, but hardly affects the f nCtlonal forces between tyre and road su rtace.
Another problem 's wIntertIme mea-sures against ICIng ,EspecIally when It is snowing, a road sectIon with a porous asphaltic concrete top layer demands extra attention.
In many cases where there IS or may be water nUIsance, the use of porous asphaltlc concrete should nevertheless be consIdered ConstructIon costs wl'l be barely hIgher than for the asphaltlc concrete mIxes generally used
ChOice of mix with a view to rutting
When the customary paving materials
are used a contInuous water fIlm WIll inevitably form during or shortly after
rainfall.
The deSIgner can limIt the thIck ress of
thIS film the mome he makes a selec -tlon from the avaIlable materials ThIS
chOIce has great influence on the speed and extent to wh ch ru S WIll for m.1t IS partIcularly In these ruts that ver ythlck
water fIlms often form. With a rut depth of 17 mm and a transverse gradIent of 2 5 per cent, the thickness of the water film in the rut IS theoretically
5
to6
mm! Such a film greatly reduces tyrel surface frictIon .(A rut of 17 mm is about the limit permitted for natIonal hIgh-ways',a transverse gradient of 2 5 per cent is preSCribed for new roads).
limItation of rutting and hence water nUIsance necessitates, among other thlrgS, more stable asphaltlc concrete mixes or even cement concrete mIxes. Macro foughness of the pavement surface
In The Netherlands, the texture depth IS usually taken as a measure of macra
-rough ress of a pavement surface T hs IS the mean depth of the cav tIes be
-tween the road surface unevennesses As thIS texture depth increases, the
v.ate rfllm on 11e pavement rurface becomes thInner, though not much ThIS is not the maIn argument ,there -fore, to aIm at great texture depth In
composing the mIx for the top laye r. Of much greater Importance IS the fact
that the system of channels this brings cOout can ensure rapId rem OIal of
water from the contact area between
tyre and road surface .When most of the water has been Jemoved from thIS a lea ,
a very thin film of water WIll remain
between the nbber of he tyre and tre
gr t on the pavement surface. In this event, the e'ltent of the frictional fa' ICes between tyre and road surface is deter
-mined maInly by the energy losses occurring by compression and re
-expansIon of the rubber of the tyre:
known as hysteresis HysteresIs h
-creases the greater the texture cepth IS.
Measurements by the State Road Laboratory show that the frictional forces between a tyre and a wet road surface are greater at all speeds and at practIcally every water-film thickness the more the texture depth increases.
MICro -roughness of the pavement surface
The extremely thin water film that may
&(1 exist on the pavement even with good macro fOughness can be broken by small sharp pOInts In the surface
The points w!'1 penetrate Into the tyre
s
rubber tread and adhesive fa ICes canthus be transferred between tyre and
surface.
The chOice of stone materials for t
re
top layer must also take into account
the decrease in surface rougnness through the pohshl'rg effect of traffic. ObvIously, measures for new roads should fIrstly be consIdered for loca
-ton sw here road users are likely to have
road sections being reconstructed, the
main attention must be pa'd to lOca
-tions where there have all'eady been
wet weather accidents.
Road maintenance
For new or reconstructed roads the latest knowledge on road desfgn and pavement technology can be taken into account both In the geometry and in
desfgning the pavement structure.
Most of the Netherlands' existing roads will not be in conformity with this atest knowledge. If imperfections in road
geometry and construction and composlli"on of the pavement structure affect the user's road safety, these fmperfectlons will have to be remedied by the road authority. The road authon' -ty need not be emp-ty-handed in
controllIng water nuisance on road
surfaces. A number of measu res will
now be reviewed.
Eliminating ruts
If the water nuisance is caused by ruts, the fOllowing action can be considerd.
1 .FI10ng.A major reqUirement for the matel1als IS that the r cad sUrface must be skid -resistant even at high speed s. Non-textured or seated -SUrface materIals do not satISfy
"liS
req lAi-e -ment wh en r 00d SUrface s a re wet.2. Planing the surface. This increasing ty
popular measure is possible if the pave
-ment has enough bearing capacity and is still construcHonally sound. But it should be remembered that rutting w·.]1
continue again afterwards.
3. Applying a new top layer. If the pave
-ment IS not only uneven but has
structural defects as well, a new top layer is apph'ed in many cases. As the new layer IS not uniformly thi'ck and the traffic load is not the same t 1T0ughou
the cross -secfon, unevenness may re
-appear very soon after the new surface is applied. ThIS depends on the stability of the paving matenal, both of the new top layer and of the pavement structure under It .If the pavement is planed before the new top layer is applied, it wil take longer fo the surface to become uneven again .It might also be con
-sidered machining away the old top
layer altogether.
Transverse dIscharge channels ensure
good water removal to the verge, especially at tOrsion sections.
A
slot IS made In the road surface and Is partly filled WItha
slurry of synthetIc resin.A
steelU
-channel IS ~xed In thiS. The transverse StrIpS on the sectional steel enable them to be fitted accurately :t ~y are removed after pla;lng. The gutters do no t perceptIbly (nterfere WIth tra"(c.Making transverse discharge channels
If water nuisance anses through flowing away along Ihes roughly parallel to the road axis, I'e. over a great length, trans -verse discharge channels may provide the answer.
Provided they are made carefully, their construction need cause no problems. The cost is very acceptable. If they are well designed, there is httle perceptible interference with traffic; so far no effect Similar to that of expansion strips on bridges has been observed. The ch an -nels Will have b be cleaned regular Iy h order to function properly.
ImproVIng macro-roughness
If a pavemen tsurface has no construc -'ona Idefects but lacks enough mac ro
-roughness, vanous meas U'es may be considered
Cuttlirg transverse grooves
is
one way. Besides Increasing frictional forces between tyre and road surface, t hs Improves the reflecting properties of (wet) pavement surfaces .The effective life of these grooves In asphaltic concrete is very brief .Espec Blly Wl'tl high traffiC volumes and If the mix contains few large stones. The life of these grooves IS, of course, muchbnger If they are made In a CEment -concrete pavement.
(Longltudina Igrooves Will also increase friction between \Ire and road surface.
lhey are Cheape r b make than trans -verse grooves. Yet thiS solution IS not
recommended because of the adverse
effects on motor cyclists' riding behavlo,U' )
Transverse grooves not only improve
the macro Toughness of
a
road surface,' hey also improve reflection .But they do not last long in asphaltl'cconcrete mixtures.As stated earlier, porous asphaltic concrete prevents continuous water films forming on the pavement surface. Besides which, it provides the surface with good macro-roughness. This latter
effect can probably also be obtained with thinner layers (2 to 3 cm) of porous asphaltic concrete. Experiments are at present being made with this.
A measure much better known than the
previous two is surface treatment. First of all, mainly for cost considerations, attention is given to treatment with a bituminous bonding agent. This is being widely used. But if not carefully done, 0 r
if the weather suddenly changes, this surface treatment may be a complete
failure. Immediately after it is app 'ed,
moreover, pebbles that are not properly bonded cause the danger of shattered windscreens. Another drawback is that the effective life of this surface treat -ment may be greatly reduced by long periods of high temperatures. Using synthetic resin as a bonding agent disposes of some of these draw -backs. Especially when a stone material with a high polishing value Is used,
permanently rough, sharp texture is obtah ed. Because of the high cost of surface treatment with synthetic resin
as a ban ding agent, its use will be limited for the time being to carefully selected
road situ ations .Intersection surfaces of tntfic aieries, for instance.
Information
Information on and along the road requires constant servicing so that, especially when weather and lighting
conditions are poor, good visibility of the boards and signs is guaranteed. Regular repair and cleaning is neces-sary. Markings have an important traffic -guidance function especially in
poor lighting and weather conditions.
The use of thermoplastic marking materials is intended to make markings clearly visible even when road surfaces are wet. Care must be taken that the markings do not interfere with
rain-water removal. A large amount of water is liable to accumulate especially on longitudinal inclines along such markings, which flows down the incline.
At such places, therefore, the marking
should consist of painted strips, unless the thermoplastic is interrupted at regular intervals. Regular road inspec -tion during rainfall or shortly after is necessary to detect such places. Such inspection is recommended for other
reasons as well. It enables places to be traced where water remains unneces -sariy on the pavement. For example, deep ruts, verges that are too high,
longitudinal and transverse seams not made with due care, and so on.
Accident
control
on
existing
roads
Accident control on existing roads when the surface is wet can be illus
-trated from a strategy developed by the Provincial Public Works Department of Noord-Brabant. The short -term objec-tive is to improve road sections where
there have been a comparatively large number of wet-weather accidents in the recent past. Based on an inspection at these locations, combined with skidding-resistance data, measures have been recommended. Detection 0 f these locations was limited to provincial roads outside built-up areas .As regards
provincial roads inside built -up areas, no skidding Tesistance data are avai
1-able, besides which the problems are more complicated.
Long -term policy is aimed at traCing and
improving road sections where there are at present few, if any, wet weather
accidents but are likely in the future.
The short term
In order to trace locations that are comparatively dangerous in wet weather ,a computer was used to find locations where there had been a wet -weather accident in 1976 (the last year for which information is available). From each such place, road "Sections with a fixed length of 2 kilometres were plotted .It was next as'certained how many accidents had happened on wet
road surfaces In each such section .In order to eliminate chance influential factors as fully as possible ,only sections with four or more we't-weather accidents were selected .To avoid dIs
-torting the picture, it was moreover checked how many of such accidents had occurred at intersections .The fol-lowing quotient was determined from the road sections selected in this way: wet-weather accidents I dry
+
wet-weather accidents . Based on thiS quotient and the absolute number of wet-weather accidents, the road sections were then classified In danger categories A to F. The least dangerous categories, E and F, were disregarded for the time being .As to those in cate -gories A to D in 1976, it was examined which categories they were In 1975andin 1974. The road sections in categones A to D in each of the three years formed a final selection of twelve road sections qualifYing for inspection.
By taking t\ elower limltforthe absolute numbe rof wet weather accidents as four, a consistent pattern was obtained for the period 1974 to 1976 .If the minimum had been put at three or less,
It'atlstical reliability would have been slighter.
The twelve locations ultimately se
-lected were inspected In November 1978, attention being paid to the follOWing charactenstics :
- type of pavement; Its condition; skidding-resistance level,
-wate rremoval;
- traffic volume and flow;
-disco rtinUlties (Intersections, exits, bus stops);
-circumstances obstructing the view. In nearly all cases a combination of various adverse characteristics was found .The conclusion was that wet-weather accidents happen especially at locations which put road users at risk even in good weather and when the surface is dry.
That the method employed is highly selective is eVident from the fact that the selected road sections represent only a very low percentage of the total road-length investigated, though about one third of all wet weather aCCidents occur there.
The medIum term
In order to trace road se etlons where there are stili few, if any, wet weather accidents ,but are likely in the future,
norms would have to be available for the follOWing fnctlon re Bted factors:
-skidding res stance;
-vanations in thiS by time and place;
-decrease In fn·etlOn at
h
aeasing speeds;-thickness of the water film 0 nthe road.
As no norms have been estabrshed for any of these factors for provinc·al roads and as research Into them wlllnot produce any results in the immediate future, a pra9matic approach must be chosen at this moment. lists can be drawn up, with the help of skidding-resistance reports, of road sections not meeting the skidding-resistance norm for national highways (0 51). For these sections, the traffic volumes, quotient of wet weather and dry
+
wet-weather accidents and also pavement type are next ascertained. A classifica
-tion can be made for each of these characteristics, each category being given a numerical rating.
A category rating is established for each road section, which is multiplied by a weighting factor. The resulting figures are totalled for each road section. That given the highest rating qualifies first for Improvement.
It should be added that the Provinc
a
I Public Works Department of Noord-Brabant has been conducting an expenment since 1976, In which skidding resistance detenoratlon of various pavements IS examined at various volumes .Furthermore, the locations where resistance measure
Depending on the volume and the trend of skidding resistance, preventive action can then be taken. This approach is now in the preparatory stage.
In the future, a fifth characteristic is to be added: the thickness of the surface water film. As data on this factor will take some time to collect, it has been
disregarded for the time being.
Mean-while the necessary information is being collected by the service engi
-neers who, after all, know their road
sections better than anyone else.
Tte followi
rg
classification can be used blr skIdding resistance: 046 -050 041 -0.45 0.36 -0.40 <036 category 1 category 2 category 3 category 4 For traffIc volume (per day) :< 5000 category 1 5,001 -6,000 ca t~ory 2 6,001 -7,000 category 3 7,001 -8,000 category 4 8,001 -9,000 category 5 9,001 - 10,000 cal~gory 6 > 10,000 category 7
For the accident quotient, categories A to Fcan be use d
as In the preceding section, figures now be'ng substitute d for letters:
category F becomes category 1 category E becomes category 2 category D becomes category 3 category C becomes category 4 category B becomes category 5 category A becomes category 6
The following categories can be used to rpavement types:
concrete category 1
asphalt category 2
paving bncks category 3
Literature
- Dijks,A .lnflue ree of tread depth of car tires on skidding resistance. Report WTHD 39 .Delft University of Tech -nology, Delft, 1972.
- Dijks,A. Wet skid res'stance of car and truck tires .Tire SC.& Techn.2 (1974) 2 (May)·.102-116.
- Dijks,A. A muit'tactor exam'nation of wet skid resis
-tance of car hes .SAE paper 741106.
- Gallaway,B.M.; Schlller, RE.& fbse,J C. The effects of rainfall intensity, pavement cross slope, surface tex
-ture and drainage lenght on pavement water depths.
Research Report 139-5. Texas Transportation Inst'!-tute .1971.
- Hanscom,F.R. Human actors In skidding'. Causation
and prevention .In: Skidding accidents; Wet-weather accident experience, human factors, and legal as
-pects. Transportation Research Record 623, P 40 47.
National Academy of SCiences, Washington, D.D, 1976.
- Hacker H J . Die Oberflachenentw8sserung von Fa hr -bahnen und ihre Bedeutung ftj'r den Strassenentwurf .
Strassenbau und Verkehrstechnik Heft 118,
1971.
- Kalender U. Abf lIss des Regenwassers von idea 1
-ebenen Fahrbahnoberflachen. Dissertation, 1971.
- Morris R S. et al. Field study of dnver visual perfor -mance during rainfall. Texas Transportation Institute, 1977 .
- Oppe S . Multlplleat ive mode Is of analysis; A descrip
-tlon and the use In analysing accident ratios as a function of houry traffic volume and road surface skidding resistance .R -77 -11 .SWOV, Voorburg, 1977 .
33pp.
Also published as :The use of multiplicative models for analysIs of road sa fety da
ta
.
Accid Anal. & Prev. 11 (1979) 2 (June); 10 1-115.- Oppe,S. The analysIs of the number of passenger cars
and lOrries involved In accidents as a function of road-surface skidding resistance and hourly traffic volume.
Paper and shortened version presented to the OECD
Symposium on Heavy Freight Vehicles and thelir
effects, Paris, 14 -16 November 197-7. R-77-12 .SWOV,
Voorburg, 1977 .18 pp. '
- Schlosser,L.H.M. Traffic accidents and road surface skidding resistance. Paper presented to the Second International Skid Prevention Conference, Columbus, Ohio, May 2-6, 1977 .R-76-16 .SWOV, Voorburg, 1976.
13 pp.
Also published In: Skidding aCCidents; Wet-weather
aCCident experience, human factors and legal aspects.
Transportation Research Record 623, pp 11-20.
National Academy of Sciences, Washington, D.C.
1976.
- Schlosser,L.H M . Tyres and road-surfaces. Paper pre-sented to the Second International Skid Prevention
Conference, Columbus, Ohio, May 2-6 ,1977 .R-76-17.
SWOV, Voorburg, 1976.13 pp.
Also published In :Skldding accidents; Ancillary papers. Transportation Research Record 624, pp .15 -26.
National Academy of Sciences, Washington, DC.,
1976.
- Schul7e,K H. et al. Skidding accidents, fnCtlon num -bers and the legal aspects liwolved. Report of the
PIARC T echmeal Committee on Shpp~nness and Even
-ness. In : Skidding accidents; Wet weather 'lccident experience, human factors and legal aspects. Trans
-portaton Research Record' 623, pp. 1 -10. National Academy of SCiences. Washington, DC, 1976.
- SCW. International Symposium on Porous Asphalt.
Rec Old 2 .Stichtlng Studlecentrum Wegenbouw SCW . Arnhem . 1976.
- SWOV (L.H M.Schlbsser) . Traffic accidents and road surface skidding resistance; An liwestigation into the
statistical relationship between the skidding resIs
-tance of the road surface and relative road risk.
Summary of the research report of Sub committe~ V
of the Working group on Tyres, Road Surfaces and
Skidding Accidents. R -75 -1 . SWOV, Voorburg, 1975.
38pp.
- SWOV (I..,H M Schl6sser) . Tyres and road surfaces;
5<perimental mU1tilllctor ·Investigation of the factors
affectlillJ the brake and s·de way forces between cal(
tyres and wet lOad surfaces. Summary, Conclusions
and Recommendations from the study by Sub
-committee I of the Working group on Tyres, Road Sur'
faces and Skidding Acc·ldents . R-7635. SWOV, Voor
-burg, 1976.47 pp.
- Welleman A.G .Water nuisance and road safety. Paper
presented to Sess~bn 11 of the 1978 OECD Symposium
on Road Drainage, Berne, 22-24 May 1978. R-78 '5.
